Conventionally, there has been known a sensor device with a sensor unit for converting a physical quantity into an electrical quantity. The sensor unit is, for example, a thermocouple. The thermocouple measures the temperature of an object by absorbing infrared rays radiated from the object.
The thermocouple indicates a temperature change amount in the form of a voltage signal. For example, if the temperature change amount is 1 degree, it is indicated in the form of a voltage signal with a voltage value of 2.3 μV.
Since a temperature change amount is indicated in the form of a voltage signal with a small voltage value, this voltage signal needs to be amplified with a high gain in order to accurately measure the temperature of an object using this voltage signal.
Accordingly, a sensor device conventionally includes an amplifier. The amplifier has a function of receiving a voltage signal from a sensor unit such as a thermocouple and amplifying this voltage signal. However, an output from the amplifier is likely to be contaminated with an offset voltage and low-frequency l/f noise inherent in the amplifier.
Chopping technology is widely known as technology for efficiently removing an offset voltage and low-frequency l/f noise inherent in an amplifier. This is technology for separating offset voltage and l/f noise parts mixed during an amplification process by the amplifier from a voltage signal. The chopping technology is, for example, disclosed in patent literatures 1 and 2 below.
Literature 1 is “Reliable High Quality Infrared Sensors have Found Their Way Into Automotive Climate Control” by Roger Diels, Melexis Product Sheet, http://www.melexis.com/Assets/Reliable_High_Quality_Infrared_Sensors_In_Automotive_Climate_Control—3810.aspx.
Literature 2 is “A CMOS Chopper Opamp with Integrated Low-Pass Filter” by A. Bakker and J. H. Huijsing, Proceedings of the 23rd European Solid-State Circuits Conference, 1997 ESS CIRC' 97.
The chopping technology is specifically described. Frequencies of offset voltage and l/f noise parts are converted into a chopping frequency and its odd number multiples, whereas the frequency of a voltage signal part indicating a temperature change is converted into a frequency in a low frequency band and even number multiples of Pre chopping frequency. In this way, the offset voltage and l/f noise parts having the chopping frequency and its odd number multiples could be removed by a low-pass filter.